1. Field of the Invention
This invention relates to telecommunications systems. It is of particular application to the digital cellular radio system known as GSM (Global System for Mobile Communications) but is not limited thereto. The invention is concerned with improving the accessibility of advanced telecommunications services.
2. Related Art
Intelligent networks (IN) have been conceived and developed in order to allow advanced and versatile telecommunications services to be supplied over conventional telephone networks. Some of these services require users to carry out transactions, using their telephones, in addition to the conditional call set-up transaction, and others require modification of the conventional call set-up transaction. Such services include the routing of calls under centralised control, according to factors such as call diversions set up by the called party, number translation services (e.g. to allow connection to different local service providers using the same number from any point within a wide area, by routing the call differently according to the origin of the call) and many other facilities, or to allow the use of special-tariff dialling codes (free, local rate, or premium). Some services require the user to enter information, subsequent to initiating a transaction. For example certain premium rate services require a special user identification number (usually known as a PIN--Personal Identification Number) to be transmitted in order to allow access to the service. This prevents personnel who have access to the telephone from making unauthorised transactions, for example international calls, or calls to premium rate services, whilst still allowing them unhindered access for other categories of transaction such as local or emergency calls.
Intelligent networks have been conceived and developed largely on the basis of an analogue Public Switched Telephone Network (PSTN), i.e. the conventional fixed telephone network. For services which require the user to enter information, subsequent to initiating the transaction, a special resource has to be switched into the connection to play a voice menu, receive DTMF tones, etc. In other words, an analogue-based exchange of information is passed between the user and the special Intelligent Network resource. In general terms the special resource then translates this into a digital signalling message to send to a intelligent network service control system for processing information on the system, such as authorising or barring the transaction attempt.
The digital cellular radio system GSM has started to develop and introduce a comparable IN development under a service description known by the acronym CAMEL (Customer Applications for Mobile Enhanced Logic). In existing proposals for this intelligent Network development, standard analogue collection of information is required in order to control service requests etc. For example, if a mobile station transmits a short-code number using e.g. DTMF tones, the switching centre recognises this as a request for an IN service and transmits this code, together with the mobile station's user identity, to a service controller which identifies the line to which the transaction relates (which may depend on the user code), and routes the transaction appropriately (or fails the transaction if the user is not authorised to use the code). This raises a particular difficulty in that in a cellular system a full speech channel is not normally allocated until a call is ultimately set up. Call set-up is carried out using a narrow-bandwidth signalling channel. This avoids allocation of a speech channel to a call attempt which is not going to succeed.
In the present specification, he term `signalling connection` is used to identify such narrow-bandwidth channels used for call set-up, et., as distinct from traffic channels.
In order that the voice menu, DTMF tones, etc. can be transmitted using the `CAMEL` system, a speech channel is required. It is inconvenient to have such channels used for service requests which will not result in a requirement for a speech channel, (e.g. if the transaction is going to fail because the correct authorisation code is not sent).
Moreover, when the mobile station is not operating on its home network, it requires that the current ("host") network can handle the IN service required. This may not be possible as different networks have different capabilities, or may use different signalling protocols, for example using the same short code to signify different services.
The GSM system also has a capability known as the Unstructured Supplementary Service Data (USSD) capability. This service was introduced to allow supplementary service control between a terminal and its home network. This capability provides transmission of a "packet" of data between the terminal and the home network and vice versa, to enable operators to introduce their own special service offerings, allowing users to operate these special services even when not operating on their home network. This capability allows such services to be introduced without the need to modify the mobile station, provided the user is informed of the sequence of keystrokes required to perform it. The system can return codes which the mobile station will recognise, e.g. error code `10` may cause the mobile station to display an appropriate message in a language selected according to the initial programming of the mobile station.
The USSD capability allows the transmission of data direct between the mobile station and the service control system, without any interaction of the switching centre. This is of particular advantage in the context of "roaming", wherein the mobile station operates with a switch belonging to an operator other than its home network. When a terminal is `roaming` the switch may only be compatible with the terminal to the extend required by the standardisation implicit in the GSM standard, and therefore services specific to the user's "home" network are not necessarily supported by the switch to which the user is currently connected.
It has not so far been possible within the standard GSM protocols to associate the sending of such message packets with a call, to manage specific call-related services in real time. USSD has only been used to update more static customer data, such as setting up a call-forward arrangement representing advice of the user's own telephone number, etc.